Photoproduction of Mesons of Quasifree Nucleons ————————————————————————– ————————————————————————– - selected results - ————————— ————————— B. Krusche, U. Basel for the CBELSA/TAPS and A2 collaborations Introduction Experimental setups Results Conclusions B. Krusche, Hirschegg, January 2014
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Photoproduction of Mesons of Quasifree Nucleons————————————————————————–————————————————————————–
- selected results -——————————————————
B. Krusche, U. Basel for the CBELSA/TAPS and A2 collaboratio ns
Introduction
Experimental setups
Results
Conclusions
B. Krusche, Hirschegg, January 2014
Structure of the Nucleon————————————————————————complex many body system
single meson production:γp → pπ, η, η′, ω...; ΣK(⋆)...
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10
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500 1000
σ[µb
]
γp→pπo
D13
P11 S11
F15
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500 1000
γp→pηS11(1535)
S11(1650)
photon energy [MeV]
multiple meson production:
N*, ∆
N(938)
N*, ∆
N* ∆
∆
N*
N*
N*,∆
N*
ππ πη
ρ σπ
π
π
η
π
η
η
π
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ang. distributions−→ dσ/dΩ
Dalitz plots−→ M(N,mi), M(m1,m2)
polarization dof:
- linearely pol. beams
- circularly pol. beams
- longitudinally pol. targets
- transversely pol. targets
- recoil polarization
−→ I⊙
−→ Σ, P , T
−→ E, G, H , F
−→ ...
elm. excitationsisospin dependent
quasifree (coherent)off the deuteron
electromagnetic excitations of the neutron————————————————————————————————————————importance of measurements off the neutron:
• different resonance contributions
• needed for extraction of iso-spin composition of elm. coupl ings
complications due to use of nuclear targets (deuteron):
• coincident detection of recoil nucleons
• Fermi motion, nuclear effects like FSI, coherent contribut ions
B. Krusche, Hirschegg, January 2014
mesurements off quasifree nucleons bound in the deuteron————————————————————————————–————————————————————————————–Complications:
(1) detection of recoil nucleons mandatory
(2) reaction kinematics modified by Fermi motion - smears out all structures
(3) possible influence of meson - nucleon and nucleon-nucleo n FSI on cross sectionsSolutions:
(1,2) Typical neutron detection efficiencies for CB and TAPS in the range 10% - 30%,CB cannot measure energies (TAPS via ToF); but kinematics co mpletely defined:
• initial state: incident photon and deuteron at restknown/measured: Eγ, md, ~pd = 0
• final state: meson, participant, and spectator nucleonknown/measured: ms, mp, Θp, Φp, mm, ~pmnot measured: Tp, ~ps (four variables)
• four constraints from energy/momentum conservation
(3) comparison of quasifree production off protons and prod uction off free protonsto study FSI effects
0
0.2
0.4
0.6
0.8
1
0 100 200 300
neutron spectator
coun
ts[a
.u.]
ps[MeV]
datasimulationdeuteron wf.
0 100 200 300 400
proton spectator
B. Krusche, Hirschegg, January 2014
MAMI accelerator in Mainz————————————–————————————–
significant deviationsbetween PWApredictions andneutron data
B. Krusche, Hirschegg, January 2014 MAMI
η-photoproduction off the proton: resonance contributions ?————————————————————————————–————————————————————————————–branching ratios and elm. couplings (PDG):
pronounced, narrow structure in neutron excitation functi on close to W=1.68 GeVwidth of structure ≈ 30 MeVneutron/proton ratios in agreement for all measurements:- in S11(1535) region 2/3 ratio- peak close to 1.7 GeV- very close to threshold almost unity, no distinction betwe en participant and spectatorfree and deuteron quasifree proton data agree;quasifree 3He data suppressed by ≈ 25%
B. Krusche, Hirschegg, January 2014
γn → nη - excitations functions for different angular bins———————————————————————————–———————————————————————————–(D. Werthmuller and L. Witthauer et al., Phys. Rev. Lett. 111 (2013) 232001; EPJA 49 (2013) 154)
photoproduction of meson pairs———————————————–———————————————–ππ- & πη-pairs: clean reaction identificationaccess to cascade decays in invariant mass spectra
N*, ∆
N(938)
N*, ∆
N* ∆
∆
N*
N*
N*,∆
N*
ππ πη
ρ σπ
π
π
η
π
η
η
π
1000 1500 2000 2500 3000
310×1000
1500
2000
2500
3000
310×
0
200
400
600
)0π(p2m
)0 π(p2
m
= 1500 - 1600 MeVγE
(1232)∆
13D (1520)
13D (1520)
1000 2000 3000 4000
310×1000
2000
3000
4000
310×
0
20
40
60
80
100
)0π(p2m
)0 π(p2
m
= 2000 - 2100 MeVγE
(1232)∆
13D (1520)
13D (1520)
15F (1680)
15F (1680)
1000 2000 3000 4000 5000
310×1000
2000
3000
4000
5000
310×
0
50
100
150
200
250
)0π(p2m
)0 π(p2
m
= 2400 - 2500 MeVγE
13D
13D (1520)
(1232)∆
15F (1680)
15F (1680)
Dalitz-plotanalysis ofπ0π0-pairs
B. Krusche, Hirschegg, January 2014 ELSA
γN → Nπoπo - total cross sections————————————————————————————————(M. Oberle et al., preliminary)
invariant mass distributions show contributions from∆⋆, N⋆ → π∆(1232) & ∆⋆, N⋆ → πD13(1520); very similar for p & n
proton & neutron angular distributions different for large W
→ different resonance contributions for ∆⋆, N⋆ → πD13(1520) ?
B. Krusche, Hirschegg, January 2014 MAMI
example for polarization observables for pion-pairs————————————————————————–————————————————————————–beam-helicity asymmetries - circularly polarized beam, un polarized target
beam-helicity asym. for γN → Nπoπo & γN → Nπoπ±——————————————————————————————————————————————————————M. Oberle et al., PLB 721(2013) 237, M. Oberle et al., submitte d to EPJA
γp → pπ0π0 (blue: qf. p , black: free p) γp → nπ0π+ (blue: qf. p , black: free p)
coefficients of sine-series compared to Fix modellarge discrepancies for second resonance regionexcellent agreement between free and quasi-freeproton data, no FSI effects
B. Krusche, Hirschegg, January 2014 MAMI
resonance contributions to photoproduction of πη-pairs—————————————————————————————————————————————————————–I. Horn et al., PRL 101 (2008) 202002; EPJA 38 (2008) 173, V. Ka shevarov et al., EPJA 42 (2009) 141; PLB 693 (2010) 551
total cross section Invariant mass distributions
dominant final states: −− ∆(1232)η, −.− N(1535)π, ... pao(980)
dominant process close to threshold: γp →D33(1700)→ ηP33(1232) → ηπop
B. Krusche, Hirschegg, January 2014 ELSA
isospin decomposition of πη-photoproduction————————————————————————————————————————————(A. Kaeser et al., preliminary)
analysis of invariant mass distributions and polarization observables for allisospin channels under way
B. Krusche, Hirschegg, January 2014 MAMI
Summary——————————measurement of final states with coincident neutrons, in par ticular‘all neutral’ final states like nπ0, nη, nη′, nπoπo... mandatory for analysisof N⋆ properties
effects from Fermi motion under control via kinematic recon struction
effects from FSI:
experimental access via comparison of free and quasi-free p roton results
develpment of models for FSI in progress
FSI effects strongly channel dependent, e.g. small/neglig ible for η, η′,moderate for πoπo, substantial for πo, ηπ
for channels so far investigated FSI effects seem to be much l ess import-ant for polarization observables than for cross sections
experiments at MAMI taking data or are under analysis,expriments at ELSA will start after detector upgrade
B. Krusche, Hirschegg, January 2014
Conclusions————————————excitation spectrum of nucleons is one of the most important testing groundsof non-perturbative QCD, but not yet understood
progress on theory side from lattice expected, but still in v ery early state
progress in experiment currently mainly from photoproduct ion of mesonsresting on three pillars:
exploraration of polarization observables (beam, target, recoil) toestablish a data base allowing almost model independent ana lyses
investigation of different final states including multi-me son productionso that coupled channel analyses can identify excited state s decoupledfrom dominant decays like π0 emission to the nucleon ground-state
investigation of reactions off quasi-free neutrons to esta blish also thephotocouplings for neutron resonances
first impact is demonstrated, resonance listings in PDG (so f ar stronglybiased to elastic pion reactions) become more and more influe nced byresults from the photon induced reactions